a) Field of the Invention
This invention relates to an imaging optical system for microscopes.
b) Description of the Prior Art
An optical microscope most suitable for observation and photometry of a cell in vivo is often utilized for conducting researches and examinations in the field of industry, not to speak of the field of biology. Recently, in keeping with the improvement of performance of various sensors including an image sensor, such as a CCD, the usage of the microscope relying mainly on visibility has changed to that relying mainly on sensors. Contrary to such a change of usage, however, the optical system and fundamental arrangement of the microscope remain unchanged, so that when the sensors are employed, it is necessary to add various improvements to the microscope in accordance with the purpose of research and the object of observation. For example, where an intermediate image located between an objective lens and an imaging lens is directly introduced to the CCD, with a visual observation of an image, to process and record an observed image through the CCD, an appreciable improvement of its mechanism is required for an optical path from the objective lens to a lens-barrel for observation, according to which a complicated optical system is also required for the security of the amount of marginal light and the maintenance of magnification.
Specifically, the use of the optical system and fundamental arrangement of the conventional microscope leads to great restriction on insertion or removal of the intermediate lens-barrel and makes it impossible to insert or remove a unit, such as an image converter intermediate lens-barrel (image module) for using the image sensor, together with intermediate lens-barrels, such as an intermediate magnification lens-barrel and a reflecting projection tube. If, therefore, the optical system is such as to be free to insert or remove two to three intermediate lens-barrels, each having a length of nearly 50 mm, it will dispense with the need for the improvements, so that a single microscope can be used in accordance with various modes of observation.
The optical system of the conventional microscope may be divided, according to the difference in its imaging system, into two classes: a finite correction type for forming an image of an object at a finite distance through the objective lens itself and an infinite correction type for changing emergent light from the objective lens to nearly parallel beams. These types are now considered from the viewpoint on whether two to three intermediate lens-barrels can be inserted or removed as mentioned above. In the former, since an image relay lens must be incorporated in the principal path of the intermediate lens-barrel (the optical path of the lens-barrel on the observation side), the provision of the intermediate lens-barrels of two or more stages complicates the optical system and the resultant increase of the number of lenses reduces imaging performance on the observation side. Thus, the intermediate lens-barrel is limited to a single stage, and two to three lens-barrels cannot be inserted. In the latter, on the other hand, light beams between the objective lens and the imaging lens become parallel, so that if the intermediate lens-barrels are interposed therebetween, the image relay lens need not be incorporated and the imaging performance will be prevented from deteriorating due to the increase of the number of lenses. If, however, a one-stage intermediate lens barrel is assumed to have a length of 50 mm, the length of a two-stage intermediate lens-barrel will be 100 mm and consequently, a variable amount of at least 100 mm must be secured between the objective lens and the imaging lens. In this case, unless aberrations, such as coma and the like, are favorably corrected, the imaging performance will likewise deteriorate. Japanese Patent Preliminary Publication No. Sho 63-311222 sets forth an imaging lens in which the variable amount of spacing where the focal length of the imaging lens is represented by f is about 0.3 f. With this imaging lens, if the value of the focal length f is 160.about.200 mm, the variable amount of spacing within the range of 48.about.60 mm will merely be secured, a distance insufficient for insertion or removal of two to three intermediate lens-barrels.